The use of reinforced plastics or composites in the fabrication of structural components has grown substantially in recent years. Composite structures are formed by overlapping layers of a "towpreg", i.e., reinforcement material such as graphite fiber impregnated with a matrix material such as epoxy. Composite structures have become increasingly popular as a replacement for metallic parts, particularly in high performance applications such as in the aircraft industry, because of the high strength to weight ratio, good corrosion resistance, good impact resistance, and high electrical and thermal resistance exhibited by composite parts.
One aspect of the composites industry which has restricted the use of composite parts in some applications is that traditionally many composite parts had to be fabricated by hand or with several manual operations. The technology of automating the formation of composite parts continues to evolve, but limitations still exist particularly in the formation of parts having a relatively complex shape, i.e., parts having contoured or arcuate surfaces as opposed to cylindrical or other standard shapes.
Early attempts to automate the formation of composite parts involved the use of filament winding machines employing a wet winding technique in which fibers of filamentary material are drawn through a resin bath mounted on a traversing carriage having a pay-out eye. A form or tool, carried on a rotating mounting structure, is located with respect to the carriage such that the resin impregnated fibers are guided under tension by the pay-out eye longitudinally along the rotating tool. The pay-out eye traverses the tool from end to end laying down successive layers of fibers until the desired wall thickness is built up on the tool. The resin or matrix material is cured on the tool, and then the tool is removed leaving the cured, wound composite structure. See, for example, U.S. Pat. Nos. 3,378,427; 3,146,926 and 3,363,849.
One advantage of filament winding machines is that the pay-out eye can be oriented with respect to the tool such that the fibers are laid down at various angles relative to the longitudinal axis of the tool. This permits the formation of a finished composite part in which the several layers of fibers forming the wall of the part are oriented in the direction in which the part will be loaded, thus providing maximum strength with minimum wall thickness. Despite this and other advantages, a number of problems or limitations are presented by current filament winding techniques. For example, in the formation of cylindrical shaped objects, the continuous fibers traverse the tool longitudinally from end to end to form the individual layers of the wall of the part. This produces a buildup of fibers at the ends of the part, compared to the center section thereof, which wastes fiber material at the ends of the part if it is not needed there.
Another problem with conventional filament winding machines relates to "compaction pressure", i.e., the pressure with which the fibers are applied onto the surface of a tool. The fibers are guided through a pay-out eye in filament winding machines and are applied to the surface of the tool under tension. The compaction pressure is dependent upon the tension on the fiber, the curvature of the surface of a tool and the width of the fibers. Tools having complex shapes such as arcuate or contoured surfaces with "peaks and valleys", i.e., concave and convex areas located adjacent one another along the winding axis, present problems for filament winding machines because the tension wound fibers span the concave surface adjacent a convex area. This is because no means are provided to press or compact the fibers directly into the concave area. The lack of direct compaction pressure between the fibers and tool surface in filament winding machines also creates problems in the winding of box-shaped parts. Because the compaction pressure is dependent, in part, on the curvature of the tool surface, the fibers are laid down on the flat sides of the box with little or no compaction whereas the corners of the box are highly compacted. The box-shaped part is thus unevenly compacted by filament winding machines, and has a thinner wall thickness around the corners than the sides.
The problem of automatically forming more complex composite parts has been solved to some extent by tape laying machines such as disclosed, for example, in U.S. Pat. Nos. 3,616,078; 4,822,444; 4,273,601; 3,775,219; 4,292,108; and 4,419,170. Machines of this type lay down a relatively wide "tape" which is essentially a pre-impregnated group of continuous individual fibers oriented parallel to one another on a carrier material. These tapes are carried in a placement head supported by structure capable of manipulating the placement head relative to a tool or form about a number of axes. Unlike filament winding devices, tape laying machines are capable of accommodating more complex shaped parts because the fibers in the tape are pressed or compacted directly onto the tool by a compaction roller or shoe carried on the placement head. The mechanisms which carry the placement head are effective to maintain the roller or shoe substantially perpendicular to the surface of the tool such that the tape is pressed against non-planar surfaces of the tool. As a result, tape laying machines are more versatile than filament winding apparatus for large, gently contoured parts and have been effective in automating the production of some parts which had previously been constructed entirely by hand or with a number of hand lay up operations.
While tape laying machines have provided an advance in the fabrication of composite parts, such machines also have limitations. One problem involves an unwanted buildup of composite layers at the small ends of a tapered tool and similar parts. There is no provision in tape laying machines for decreasing the numbers of fibers within the tape as the placement head reaches the smaller ends of a tapered tool, for example, and therefore more fiber material can be built up on the ends than the center of the tool.
Another problem with tape laying machines is that they are incapable of laying down the tape along an arcuate or curved path except where the arc or angle of the path is extremely large. As mentioned above, the tape consists of fibers oriented parallel to one another on a carrier material. If the placement head of the tape laying machine is moved in an arcuate path, the tape tends to wrinkle or buckle because all of the fibers in the tape are of the same length. In order for a tape laying machine to accommodate arcuate paths, the fibers along one edge of the tape must subtend a different length than those on the opposite edge so that the tape conforms to such an arcuate path. Variation in the length of the fibers within the tape is not possible in currently available tape laying machines.
A third generation of automated devices for the fabrication of composite parts is disclosed, for example, in U.S. Pat. No. 4,699,683 to McCowin. Apparatus of the type disclosed in the McCowin patent are referred to as "fiber placement" machines and differ from tape laying machines in that they apply a number of individual fiber tows side-by-side onto a form or tool rather than a pre-formed tape that is reeled with a carrier material. Fiber placement machines include a creel assembly consisting of a number of spools of pre-impregnated fibers, known as towpregs, which are individually fed at independently controlled rates to a fiber placement head. The fiber placement head includes structure for handling each tow individually. This structure is effective to feed the several tows side-by-side to form a fiber band which is pressed onto the surface of the tool by a compaction roller or shoe. The fiber placement head also includes structure for individually cutting one or more of the tows so that they can be "dropped off" from the remaining tows being applied to the tool.
The ability to selectively cut individual tows within the fiber band has a number of advantages. One advantage of selectively cutting individual tows is that the fiber placement head can lay down the tows in an arcuate path. This is because the length of the individual tows can vary since each individual tow is allowed to subtend a different line length compared to adjacent tows forming the fiber band. Another advantage of cutting individual tows is that material savings are obtained in forming tapered parts and the like wherein one or more of the fibers can be "dropped off" or cut as the fiber placement head reaches the ends of the tool to avoid unwanted buildup of fiber thereat. A still further advantage of permitting cutting of each tow individually is that "windows", e.g., holes, cut-outs, etc., formed in the tool can be accommodated by dropping off one or more tows as the fiber placement head moves past so that the windows are uncovered or free of fiber material.
The apparatus disclosed in the McCowin U.S. Pat. No. 4,699,683 provides distinct improvements over tape laying machines because of its capability to independently feed and cut a number of individual fibers which form the fiber band. Nevertheless, a number of limitations have been encountered with the McCowin apparatus, particularly in the construction of its fiber placement head.
One problem with the fiber placement head disclosed in the McCowin U.S. Pat. No. 4,699,683 relates to the mechanisms for cutting and clamping each individual tow in the event it is desired to "drop off" one or more tows from the fiber band. As mentioned above, a number of individual tows are fed at independent rates from a creel assembly to the fiber placement head of the McCowin apparatus. If it is desired to remove one or more of the tows from the side-by-side fiber band, such tows must first be cut and then the cut end must be clamped in place until such time as movement of the cut tow to the compaction roller is resumed. In the McCowin apparatus, a separate knife blade for each individual tow is operatively connected to a cylinder which extends the knife blade to shear the tow, and then retracts the knife blade once the cutting operation is completed. A clamping mechanism is located upstream from the knife blade which is operated by a separate, second cylinder. After the knife blade has cut the tow, the clamping mechanism is operated by the second cylinder to clamp or pinch the tow against a rethread roller.
The problem with the design of the McCowin cut and clamping mechanisms described above is that they operate independently of one another, i.e., one cylinder operates the knife blade and a second cylinder operates the clamping mechanism. As a result, the operation of the two cylinders must be carefully synchronized so that the cutting and clamping operations are performed in sequence and at the appropriate time intervals. This complicates the design and operation of the fiber placement head of the McCowin apparatus.
Another problem with the McCowin fiber placement head is that advancement of the tows to the compaction roller after they have been cut is controlled by rotation of the compaction roller. As mentioned above, once a tow is cut, the clamping mechanism presses the cut end of the tow against a rethread roller which must be rotated to advance or rethread the tow. This rethread roller is drivingly connected to the compaction roller such that the rethread roller is rotated only upon rotation of the compaction roller. As a result, the compaction roller must be in contact with the surface of the tool or form and rotate therealong over at least some distance before the tow is advanced into a position beneath the compaction roller. This causes a delay in returning the cut tow into position, thus resulting in a temporary "drop off" of one tow at a location where such might not be desired.
Another problem with the fiber placement head disclosed in McCowin U.S. Pat. No. 4,699,683 is that it is ineffective to sufficiently cool the individual tows in the course of their movement through the fiber placement head. As mentioned above, the composite tows typically comprise graphite fibers impregnated with an epoxy matrix material. Epoxy and other matrix materials can become "tacky" even at ambient temperatures which makes them difficult to smoothly feed through the fiber placement head. In the McCowin apparatus, a single tube carrying cooling air is positioned a short distance upstream from the compaction roller for the purpose of cooling the tows as they are fed beneath the compaction roller. It is believed that this structure is ineffective to sufficiently cool the tows, particularly where a large number of tows are laid side-by-side to form the fiber band. Additionally, the cooling air is directed onto the tows downstream from the clamping and rethread mechanisms of the fiber placement head in the McCowin apparatus and thus does not ensure the smooth transit of the tows therethrough.
A further limitation of the apparatus disclosed in McCowin U.S. Pat. No. 4,699,683 is that it requires the use of a compaction roller to force the fiber band against the surface of a tool. A compaction roller rotates with the movement of the fiber placement head relative to the tool, and therefore applies little or no shear force to the tows. Such rollers, however, are rigid across their entire width and cannot conform to the surface of the tool. If a surface irregularity, e.g., a convex or concave area, is formed in the tool then the tows may not be firmly pressed against such surface by the roller.
This problem has been addressed in tape laying machines wherein a friction or compaction shoe is substituted for the compaction roller and is capable of conforming to an irregular surface of the tool. These friction shoes are generally constructed of a number of individual sections mounted side-by-side which are vertically movable relative to one another to conform to most shapes. The problem with friction shoes is that they slide along the surface of the tape, and would damage the tows if permitted to directly contact them. The McCowin apparatus provides no means for protecting such tows from frictional engagement with a compaction shoe or similar mechanism, and thus a compaction roller is utilized therein.